A method to taking a tomograpic image of human tissue noninvasively by using low coherent interference is known (for example, Patent Literature 1). In the tomograpic imaging, low coherence light emitted from a light source is divided into irradiation light and reference light. The irradiation light is irradiated to human tissues through a light guide. On the other hand, the reference light enters a movable mirror in an optical path length adjusting section. After that, the irradiation light (scattered light) reflected by respective tissues of a human body and the reference light reflected by the movable mirror are combined, and reference light having the same optical path length as the optical path length of the reference light is participate in interference at that place. Accordingly, by analyzing the intensity of interfering light, there can be obtained a tomographic image of tissues corresponding to scattered light having the optical path length which is the same as the optical path length of the reference light. By using the theory of the optical tomography, a tomographic image of tissues which spread in a broad area can be obtained. For example, an optical fiber wherein a rectangular prism is arranged on its leading end is inserted in a blood vessel, and the optical path length of the reference light is changed corresponding to the tomographic imaging position while scanning is carried out with the irradiation light in the circumference direction by rotating the optical fiber on the longitudinal axis. Thereby, a tomographic image of a blood vessel can be obtained.
As described above, in the optical tomography in which scanning is carried out with the irradiation light in the circumference direction, a light guide tube for guiding the irradiation light which has been separated from the reference light is composed of, for example, an optical fiber (non-rotating optical fiber) which guides the irradiated light which has been separated and a second optical fiber (rotary scanning optical fiber) which receives the irradiated light from the former optical fiber to guide the light to the rectangular prism at the leading end and rotates on the longitudinal axis. In this case, it is required that the first optical fiber and the second optical fiber are optically connected together with the second optical fiber being rotatable with respect to the first optical fiber.
Patent Literature 2 discloses an optical connector which meets the above demands. The optical connector of Patent Literature 2 employs a structure that plural optical lenses are arranged between the end section of the first optical fiber and the end section of the second optical fiber and light emitted from one of the end section of the optical fibers is converged by the lenses to enter into the other end section of the optical fibers. However, as for the optical connector, it is required that small lenses are mounted in a small area with accuracy.
Another optical connector which solves the defect of the lens-type connector disclosed in Patent Literature 2, is disclosed, for example, in Patent Literature 3. The optical connector disclosed in Patent Literature 3 connects the ends of the first and second optical fibers by closing the ends in a narrow space in a tube shape, whereby the ends are optically connected without using lenses. Refractive-index matching liquid is deposited in the gap between the ends to reduce Fresnel reflection that light is reflected on the end surfaces of the optical fibers.
However, a rotating optical fiber oscillates in the longitudinal direction corresponding to its rotation, which changes the distance of the opposite surfaces of the two optical fibers. This change in distance is propagated to the matching liquid filling the space between the optical fibers, which causes a change of the liquid in pressure and changes of the liquid in density and refractive index coming from the change in pressure. The change in refractive index brings generation of noise and distortion of the tomographic image, which are problems.
Further, when negative pressure is applied to the matching liquid, bubbles are generated or flows in the matching liquid and the light propagating efficiency is significantly reduced because the bubbles block an optical path. Concretely, in the case of a single-mode optical fiber, the outer diameter of a clad is about 125 μm and the pressure of the matching liquid considerably changes when the interval of the two optical fibers changes by only 1 mm in the direction of the longitudinal axis, which can make bubbles easily. The generation of bubbles causes the problem that a tomograpic image cannot be formed.